In recent years, along with the trend for achieving higher packing density in integrated circuit semiconductor devices, active research and development in the field of isolation techniques has been in progress, striving for device miniaturization. Therefore, isolation regions, which are necessary to prevent current flow between bipolar devices on a single substrate, taking up a considerable portion of a chip, should be reduced in order to proportionally shrink the size of the semiconductor devices over the whole chip pattern.
Conventionally, the LOCOS (LOCal Oxidation of Silicon) method has been generally utilized as one of the isolation techniques for forming isolation regions on a substrate.
The conventional process for forming a field oxide layer by the LOCOS method is explained with reference to FIGS. 1A to 1D. In FIG. 1A, a pad oxide layer 2 is formed over a silicon substrate 1, and a non-oxidizable silicon nitride layer 3 is then formed thereon. Whereafter, as shown in FIG. 1B, a photoresist 4 is applied on the nitride layer 3, and channel stop regions 7 are formed by ion implanting an impurity of the same conductivity type as that of the silicon substrate 1. As shown in FIG. 1C, after removing the photoresist 4, field oxide layers 5 are formed by oxidation for defining an isolation region. Finally, the isolation region as shown in FIG. 1D is formed by removing the nitride layer 3 and the pad oxide layer 2.
The main characteristic of the aforesaid method is that the impurity is doped to form the channel stop layer on the isolation region by self-alignment which is used as a technique for mass production of semiconductor devices commonly designed on a 1 .mu.m grid.
However, it is the principle problem of the method that a portion of the field oxide layer called a "bird's beak" intrudes into the active element regions from the isolation region during selective oxidation, causing the isolation region to have an increased size. Although reduction of the bird's beak region can be achieved by making the field oxide layer a thin film, the thin-filming of the field oxide layer restricts miniaturization in the sub-micron region.
Accordingly, methods of forming isolation regions which reduce the bird's beak size have been actively studied in recent years.
One approach of the studies is the improvement of a selective oxide layer, and SWAMI (Side Wall Masked Isolation) and SEPOX (SElective Polysilicon OXidation) can be cited as the typical methods for the improvement. Another approach is to form a groove filled with an insulating material, and BOX (Buried OXide isolation) can be cited as the typical method thereof.
FIGS. 2A to 2D show the process for forming a field oxide layer manufactured by the SWAMI method.
Referring to FIG. 2A, after a first pad oxide layer 11 and a first nitride layer 12 of Si.sub.3 N.sub.4 are formed over a silicon substrate 10, the first nitride layer 12 and the first pad oxide layer 11 are selectively etched. Successively, the exposed silicon nitride substrate 10 is etched to a predetermined depth. Thereafter, an impurity of the same conductivity type as that of the silicon substrate 10 is ion implanted into the exposed silicon substrate 10 by using the remaining nitride layer 12 as a mask, thereby forming a channel stop region 13. As illustrated in FIG. 2B, a second pad oxide film 14 is then grown on the exposed silicon substrate 10, and a second nitride layer 15 is deposited over the whole surface of the resultant structure. An oxide layer 16 is then thickly deposited thereon. As illustrated in FIG. 2C, spacers 17 are formed by anisotropically etching the oxide layer 16 and the second nitride layer 15 and to expose the first nitride layer 12. The field region is continuously oxidized to form a thick field oxide layer 18, as illustrated in FIG. 2D.
However, in the aforedescribed SWAMI method, the manufacturing process of the spacer 17 formed for preventing the formation of the bird's beak of the field oxide layer 18 is fastidious. Also, etching of the silicon substrate may create defects in the silicon substrate. In addition, since the channel stop region is impurity-doped before forming the spacer, it is disadvantageous in that the edge portions of the channel stop region may expand into the active region resulting in the lowering of the junction breakdown voltage of the device. Therefore, the impurities can not be heavily doped into the channel stop region.
FIGS. 3A to 3D show the :process for forming a field oxide layer manufactured by the conventional SEPOX method.
Referring to FIG. 3A, a pad oxide layer 21 is grown by thermal oxidation on a silicon semiconductor substrate 20, and a polysilicon layer 22 and a nitride layer 23 are sequentially formed over the pad oxide layer 21. Referring to FIG. 3B, the nitride layer 23 is etched via reactive ion etching using a photoresist 24 as a mask, thereby forming a pattern. Thereafter, an impurity having the same conductivity type as that of the substrate is ion implanted through the polysilicon layer 22. Referring to FIG. 3C, the photoresist 24 is removed, and the exposed polysilicon layer 22 is thermally oxidized for forming field oxide layers 26. Referring to FIG. 3D, after the nitride layer 23 is removed, the unoxidized polysilicon layer 22 is etched via reactive ion etching. Referring to FIG. 3E, the leftover tips of polysilicon of the polysilicon layer 22 which remain from the previous step, are oxidized as in the SWAMI method, thereby planarizing the end portion of the field oxide layer.
However, in the aforedescribed SEPOX method, since a field oxide layer is formed by thermally oxidizing a polysilicon layer formed over a silicon substrate, it is disadvantageous in that the step coverage is degraded. In addition, since the channel stop region is formed to be self-aligned with the active region, the breakdown voltage of the channel stop layer is lowered. As a result, the channel stop layer cannot be heavily doped, and problems such as punch-through have arisen.